Small, engine driven earth cultivators or tilling machines having ground engaging, rotating tines and handlebars manipulated by an operator following the machine on foot, have been in use a number of years. Normally, a plurality of tine members are mounted on a rotatable shaft to form a transversely extending tine wheel assembly mounted beneath the machine's engine. The rotating tines are urged to penetrate the ground under the weight of the machine and create a forward traction force to propel the machine forward as the earth is being tilled. The rate at which the machine is propelled forward is dependent on several factors including the number and configuration of the tines, the rate at which the tine wheel rotates, the weight of the machine, and the nature and composition of the soil being tilled. Usually, in the interest in manufacturing economy, the tine wheel is driven at a single speed so that the only variable factor affecting the forward speed of tilling machine is the nature and composition of the soil. In the case of hard, densely packed soil, the tilling machine moves more rapidly over the ground due to the fact that the tines experience additional traction as a result of increased resistance presented by the packed soil to the movement of the tines therethrough. Also, because of the additional resistance presented by the soil, the tines do not fully penetrate the soil in their course of rotation, but rather push the machine upwardly away from the ground. Obviously, under these conditions, the tilling machine tends to move rapidly over upper surface layers of the ground and does not thoroughly till the soil in the desired manner.
As evidenced by U.S. Pat. No. 2,824,506, others in the past have attempted to ameliorate this problem by rigidly mounting a plow-like braking tool on the tilling machine which functions to plow into the soil and produces a negative braking force to counteract the excess forward thrust developed by the tine wheel, resulting in the forward speed of the machine being slowed so that successive revolutions of the tines dig down into the soil to achieve penetration to the desired soil depth and thorough tilling action. Generally, during the tillage of a typical plot of ground, the density and hardness of the soil will vary in adjacent locations, consequently, the braking tool may operate satisfactorily in the more densely packed soil locations, but exerts an excessive amount of braking force when the tilling machine enters areas of less dense soil, in which case the thrust produced by the tine wheel is not sufficient to propel the machine forward and the tine wheel proceeds to dig successively deeper into the soil until the tilling machine becomes stalled in one location. Most known tilling machines are provided with a frame arrangement including elongate handlebars which allows pivotal movement of the machine by its operator in a manner to permit altering the tine wheel's elevation, so that by selective manipulation of the handlebars in combination with the use of the braking tool, the effects on the machine produced by varying soil conditions are at least partially neutralized. In practice, however, the use of these plow-like braking tools results in the tilling machine lurching forward from time to time as it becomes impossible to eliminate momentary lurching and stalling of the machine; in fact, the operation of the tilling machine resulting from the use of the braking tool is sufficiently unsatisfactory that it is not uncommon for many operators to remove and discard the braking tool. Without the use of a braking tool, the vehicle operator must compensate for the excessive thrust produced by the tine wheel by pulling rearwardly on handlebars to counter-act the thrust, while at the same time selectively pivoting the machine in order to maintain a constant penetration depth.
Another problem complicating the design of tilling machines, related somewhat to the foregoing problems, is the difficulty in cultivating loosely packed or low density soil. When tilling low density soils using conventional tilling machines, the soil offers little resistance to the tines, consequently the traction created by the tine wheel sometimes is inadequate to propel the machine forward in which case the tines dig successively deeper into the soil and the machine becomes stalled in one location unless the operator tilts the machine in a manner to raise the tine wheel before the latter digs too deeply into the ground. Since it is impractical for the operator to actually push the tilling machine forward when inadequate thrust is provided by the tine wheel in loose soil, others in the past have designed tilling machines having a ground engaging drive wheel, in addition to the tine wheel, which is driven by the engine and functions to assure that the machine is provided with forward propulsion at all times, regardless of the nature and composition of the soil being tilled. In addition to the previously mentioned prior art patent, further prior art disclosing tilling machines having a power driven drive wheel includes U.S. Pat. Nos. 2,645,987 and 2,776,533. Although these prior art tilling machines compensate somewhat for the lack of tine wheel thrust under loose soil conditions, these machines have been found not to operate satisfactorily when the ground is relatively hard, even in conjunction with the use of a plow-like braking tool discussed above. Typically, for reasons which will become later apparent, the prior art tilling machines employed belt and pulley assemblies commonly connecting the tine wheel and a single ground engaging drive wheel with the engine, and as result of slippage, along with other factors, the tine wheel and ground engaging drive wheel were not locked into constant, positive driving relationship to each other, consequently, when traversing harder ground, the excessive thrust created by the tine wheel could not be counter-acted by the ground engaging drive wheel, but rather the plow-like braking tool was required to compensate for the excessive forward thrust. Also, under some circumstances, due to the excessive resistance created by solidly packed soil, slippage in the drive belt assembly between the engine and tine wheel often resulted so that proper tilling action could not be achieved. Furthermore, when this type of prior art tilling machine was employed to till loosely packed, low density soil, wherein the tine wheel created negligible forward driving thrust, the ground engaging drive wheel often experienced slippage in the drive belt assembly which prevented the delivery of a constant, metered driving force needed to propel the machine forward at a constant rate of speed. In any event, it can be appreciated from the foregoing discussion that as a result of the rapidly changing magnitudes of thrust experienced by the tine wheel when the latter traversed soils of various hardness the drive belt assembly experienced frequent slippage due to the impulse-like transmission of forces transmitted therethrough, consequently, the ground engaging drive wheel was ineffective to supply both the forward and reverse traction forces in the proper magnitudes and at the proper times to permit smooth, satisfactory operation of the machine, so that the operator was still required to selectively manipulate the machine's handlebars in order to compensate for the lurching and stalling effects experienced by these machines.
A further undesireable feature of the prior art tilling machines of the type having a single ground engaging, power driven wheel resides in the fact that the tine wheel was required to be mounted at a position laterally offset from the longitudinal axis of the machine, on the opposite side of the wheel being driven in order to compensate for the torquing effect produced by the driving of only one, rather than both ground engaging wheels. This construction feature of prior art machines was unattractive from a manufacturing standpoint and necessarily limited the width of the tine wheel which could be used. Apparently, prior art machines included provision for driving only one of the ground engaging wheels, since in order to permit sharp turning of the machine, one of the ground engaging wheels was required to be free wheeling and acted as a pivot point about which the opposite driving wheel propelled the machine during the execution of a turn of the latter. Although these prior art machines satisfactorily executed sharp turns, tractability was poor as a result of the interplay of torquing forces produced by the tine wheel and ground engaging drive wheel. Because of the deficiencies inherent in the prior art machine design of the type having a single, power driven ground engaging wheel, tilling machines employing such designs have never gained widespread use and the vast majority of present design tilling machines do not include a power driven drive wheel.